1. Field of the Invention
This invention relates to a method and apparatus for dewatering ultra-fine coal and more particularly to dewatering an aqueous coal slurry after separating peptized clay from coal particles in a slurry thereof.
2. Description of the Prior Art
In U.S. Pat. No. 4,537,599 there is disclosed a process for removing sulfur and ash, particularly clay and pyrite from the surface of coal particles. A peptized slurry of coal particles is treated to separate clay and pyrite from the coal and weaken chemically bonded contaminants on the coal surface. The clay and pyrite particles are dispersed as a colloid in an aqueous medium of the slurry. To maintain the colloidal suspension, the pH of the slurry is adjusted by the addition of a normalizer. The slurry is then beneficiated in a centrifuge and in froth-flotation cells to recover coal particles greater than two microns. Thereafter, an aqueous coal slurry is again formed and the pH is adjusted to maintain contaminants as colloids in the aqueous medium of the slurry during treatment with sonic energy and ozone. Thereafter, the aqueous coal slurry with the aqueous medium containing colloidal contaminants is classified to again separate coal particles from contaminants.
The cleaned coal product recovered from this process can be used for diverse purposes, one use as disclosed in U.S. Pat. No. 4,662,894, is as a feed stock for the preparation of a coal water mixture. The cleaned coal product is first separated so that coal particles of different size classifications form two or more coal feed streams which are delivered to separate surge vessels in a liquid medium. The feed stream comprised of coal particles having the smallest size are again classified to discard a minus two micron fraction which is comprised mainly of contaminants, particularly clay, and thereafter selected quantities of each of the coal feed streams are mixed together in the presence of a dispersing agent to form a coal-water mixture.
It is time consuming and costly, particularly in terms of energy requirements, to reduce the water content of a mass of coal particles, particularly in instances where the coal particles after cleaning to remove clay and pyrite are to be used in the form of a feed stock having a low moisture content for any of diverse purposes such as for a coal-water mix. The moisture content of the cleaned coal after classification in the centrifuge according to the process of U.S. Pat. No. 4,537,599, is usually about 32% to 36% by weight and no significant moisture reduction occurs even after several days residence in storage. When an aqueous coal water slurry is dewatered in a conventional belt press, it was found that the moisture content was about 36%.